Piezoelectric laminate, surface acoustic wave device, thin-film piezoelectric resonator, and piezoelectric actuator

ABSTRACT

A piezoelectric laminate including a base and a first piezoelectric layer formed above the base and including potassium sodium niobate. The first piezoelectric layer is shown by a compositional formula (K a Na 1-a ) x NbO 3 , “a” and “x” in the compositional formula being respectively 0.1&lt;a&lt;1 and 1≦x≦1.2.

This application is a divisional of U.S. patent application Ser. No.11/634,009 filed on Dec. 5, 2006. This application claims the benefit ofJapanese Patent Application No. 2006-113494 filed Apr. 17, 2006, andJapanese Application No. 2005-351955 filed Dec. 6, 2005. The disclosuresof the above applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a piezoelectric laminate including apotassium sodium niobate layer, and a surface acoustic wave device, athin-film piezoelectric resonator, and a piezoelectric actuatorincluding the piezoelectric laminate.

A demand for a surface acoustic wave device has rapidly increased alongwith a remarkable development in the communication field represented bymobile communication such as a portable telephone. The development ofthe surface acoustic wave device has been trending toward a reduction insize and an increase in efficiency and frequency. This requires a higherelectromechanical coupling factor (coefficient) (k²), more stabletemperature properties, and a higher surface acoustic wave propagationvelocity.

A surface acoustic wave device has been mainly used which has astructure in which interdigital transducers are formed on apiezoelectric single crystal. As typical examples of the piezoelectricsingle crystal, a rock crystal, lithium niobate (LiNbO₃), lithiumtantalate (LiTaO₃), and the like can be given. For example, LiNbO₃ witha high electromechanical coupling factor is used for an RF filter forwhich an increase in band and a decrease in loss in the passband arerequired. A rock crystal with a small temperature coefficient offrequency is used for an IF filter for which stable temperatureproperties are required in a narrow band. LiTaO₃ with anelectromechanical coupling factor and a temperature coefficient offrequency between those of LiNbO₃ and a rock crystal plays anintermediate role between LiNbO₃ and a rock crystal. In recent years, acut angle of a potassium niobate (KNbO₃) single crystal showing a highelectromechanical coupling factor has been found. A KNbO₃ single crystalplate is disclosed in JP-A-10-65488.

In a surface acoustic wave device using a piezoelectric single crystalbase, properties such as the electromechanical coupling factor,temperature coefficient, and speed of sound are specific to the materialand determined by the cut angle and the propagation direction. Forexample, a 0°Y—X KNbO₃ single crystal base has an excellentelectromechanical coupling factor, but does not show zero temperatureproperties at or near room temperature, differing from a 45° to 75°rotated Y—X KNbO₃ single crystal base.

SUMMARY

According to a first aspect of the invention, there is provided apiezoelectric laminate comprising:

a base; and

a first piezoelectric layer formed above the base and includingpotassium sodium niobate.

According to a second aspect of the invention, there is provided asurface acoustic wave device comprising the above-describedpiezoelectric laminate.

According to a third aspect of the invention, there is provided athin-film piezoelectric resonator comprising the above-describedpiezoelectric laminate.

According to a fourth aspect of the invention, there is provided apiezoelectric actuator comprising the above-described piezoelectriclaminate.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross-sectional view schematically showing a firstpiezoelectric laminate according to one embodiment of the invention.

FIG. 2 is a cross-sectional view schematically showing a piezoelectriclayer of a piezoelectric laminate according to one embodiment of theinvention.

FIG. 3 is a cross-sectional view schematically showing anotherpiezoelectric layer of a piezoelectric laminate according to oneembodiment of the invention.

FIG. 4 is a cross-sectional view schematically showing a modification ofthe first piezoelectric laminate according to one embodiment of theinvention.

FIG. 5 is a cross-sectional view schematically showing a secondpiezoelectric laminate according to one embodiment of the invention.

FIG. 6 is a cross-sectional view schematically showing a modification ofthe second piezoelectric laminate according to one embodiment of theinvention.

FIG. 7 is a graph showing the hysteresis characteristics of apiezoelectric layer in an example according to the invention.

FIGS. 8A and 8B show SEM images of the surfaces of piezoelectric layersaccording to an example of the invention, and FIG. 8C shows an SEM imageof the surface of a piezoelectric layer according to a comparativeexample.

FIG. 9 shows an XRD image according to an example of the invention and acomparative example.

FIG. 10 shows the field-strain relationship of a piezoelectric layeraccording to an example of the invention.

FIG. 11 shows the SAW oscillation waveform of a piezoelectric layeraccording to an example of the invention.

FIG. 12 shows Raman spectroscopy results according to an example of theinvention.

FIG. 13 shows the relationship between the amount of excess Na and thespacing according to an example of the invention.

FIG. 14A shows an SEM surface image according to an example of theinvention, and FIG. 14B shows an SEM surface image according to areference example.

FIG. 15A shows an XRD image according to an example of the invention,and FIG. 15B shows an XRD image according to a reference example.

FIG. 16 is a view schematically showing a surface acoustic wave deviceaccording to one embodiment of the invention.

FIG. 17 is a view schematically showing a frequency filter to which isapplied a surface acoustic wave device according to one embodiment ofthe invention.

FIG. 18 is a view schematically showing an oscillator to which isapplied a surface acoustic wave device according to one embodiment ofthe invention.

FIG. 19 is a view schematically showing a first thin-film piezoelectricresonator according to one embodiment of the invention.

FIG. 20 is a view schematically showing a second thin-film piezoelectricresonator according to one embodiment of the invention.

FIG. 21 is a view schematically showing an inkjet head to which isapplied a piezoelectric actuator according to one embodiment of theinvention.

FIG. 22 is a perspective view schematically showing an inkjet head towhich is applied a piezoelectric actuator according to one embodiment ofthe invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

The invention may provide a piezoelectric laminate in which a potassiumsodium niobate layer is formed on a base.

The invention may also provide a surface acoustic wave device and athin-film piezoelectric resonator including the above piezoelectriclaminate.

The invention may further provide a piezoelectric actuator including theabove piezoelectric laminate.

According to one embodiment of the invention, there is provided apiezoelectric laminate comprising:

a base; and

a first piezoelectric layer formed above the base and includingpotassium sodium niobate.

In this piezoelectric laminate, the first piezoelectric layer may beshown by a compositional formula (K_(a)Na_(1-a))_(x)NbO₃, “a” and “x” inthe compositional formula being respectively 0.1<a<1 and 1≦x≦1.2.

In this piezoelectric laminate, “x” in the compositional formula of thefirst piezoelectric layer may be 1<x≦1.1.

In this piezoelectric laminate, an orientation control layer may beformed below the first piezoelectric layer.

In this piezoelectric laminate, the orientation control layer mayinclude nickel lanthanate. The nickel lanthanate may be polycrystalline.

The piezoelectric laminate may further comprise:

a second piezoelectric layer continuous with the first piezoelectriclayer and including potassium sodium niobate,

wherein the second piezoelectric layer may include an element forming alayer which contacts the second piezoelectric layer and is positionedopposite to the first piezoelectric layer.

In this piezoelectric laminate, the first piezoelectric layer mayinclude:

a first-phase portion including a piezoelectric of a compositionalformula (K_(a)Na_(1-a))NbO₃; and

a second-phase portion including a piezoelectric of a compositionalformula (K_(a)Na_(1-a))_(x)NbO₃, in which “x” is larger than 1.

The piezoelectric laminate may further comprise an electrode formedabove the first piezoelectric layer.

The piezoelectric laminate may further comprise:

a first electrode formed between the base and the first piezoelectriclayer; and

a second electrode formed above the first piezoelectric layer.

According to one embodiment of the invention, there is provided asurface acoustic wave device comprising the above-describedpiezoelectric laminate.

According to one embodiment of the invention, there is provided athin-film piezoelectric resonator comprising the above-describedpiezoelectric laminate.

According to one embodiment of the invention, there is provided apiezoelectric actuator comprising the above-described piezoelectriclaminate.

Embodiments of the invention will be described below with reference tothe drawings.

1. PIEZOELECTRIC LAMINATE

1.1. First Piezoelectric Laminate

FIG. 1 is a cross-sectional view schematically showing an example of afirst piezoelectric laminate 100 according to this embodiment.

The first piezoelectric laminate 100 includes a base 1, a piezoelectriclayer 3 formed on the base 1 and including potassium sodium niobate, andan electrode 4 formed on the piezoelectric layer 3.

The base 1 is selected depending on the application of the piezoelectriclaminate 100. The material and the structure of the base 1 are notparticularly limited. As the base 1, an insulating substrate, asemiconductor substrate, or the like may be used. As the insulatingsubstrate, a sapphire substrate, an STO substrate, a plastic substrate,a glass substrate, or the like may be used. As the semiconductorsubstrate, a silicon substrate or the like may be used. The base 1 maybe a single substrate or a laminate in which another layer is stacked ona substrate.

As shown in FIG. 4, an orientation control layer 6 may optionally beprovided on the base 1. The orientation control layer 6 is called abuffer layer or a seed layer, and has a function of controlling thecrystal orientation of the piezoelectric layer 3. Specifically, thepiezoelectric layer 3 formed on the orientation control layer 6 isprovided with a crystal structure similar to the crystal structure ofthe orientation control layer 6. As the orientation control layer 6, acomplex oxide having a crystal structure similar to that of thepiezoelectric layer 3 may be used. For example, a perovskite oxide suchas nickel lanthanate (LaNiO₃) may be used as the orientation controllayer 6. Nickel lanthanate may be polycrystalline. It suffices that theorientation control layer 6 control the orientation of the piezoelectriclayer 3. The orientation control layer 6 may have a thickness of 50 to100 nanometers, for example.

The piezoelectric layer 3 includes a piezoelectric of the compositionalformula (K_(a)Na_(1-a))_(x)NbO₃. In the above compositional formula, “a”is preferably 0.1<a<1, and more preferably 0.2≦a≦0.7, and “x” ispreferably 1≦x≦1.2, and more preferably 1<x≦1.1. The piezoelectric ofthe compositional formula (K_(a)Na_(1-a))_(x)NbO₃ has an orthorhombiccrystal structure at room temperature. If “a” in the above compositionalformula is within the above range, the phase change temperature from anorthorhombic crystal to a rhombohedral crystal (a≦0.55) and the phasechange temperature from an orthorhombic crystal to a monoclinic crystal(0.55≦a) are decreased to −40° C. or less, whereby stablecharacteristics can be obtained in a low temperature region. If “a” is0.1 or less, a heterophase occurs during crystallization heat treatmentdue to volatilization of potassium, whereby characteristics such aspiezoelectric characteristics and ferroelectric characteristics areadversely affected. If “x” is within the above range, since the crystalis formed at a low temperature, volatilization of potassium issuppressed, whereby the layer density is improved.

In this embodiment, the piezoelectric layer 3 may be a homogeneouslayer, or may be a layer having a structure as shown in FIG. 2 or 3.FIGS. 2 and 3 are conceptual or schematic views.

As shown in FIG. 2, the piezoelectric layer 3 may include a firstpiezoelectric layer 32 including potassium sodium niobate of thecompositional formula (K_(a)Na_(1-a))NbO₃, and a second piezoelectriclayer 34 formed between the first piezoelectric layer 32 and the base 1and including a piezoelectric containing at least an element forming alayer (base 1 in the example shown in FIG. 2) contacting thepiezoelectric layer 3. The second piezoelectric layer 34 may includepotassium and sodium (A-site elements) in excess in comparison with thefirst piezoelectric layer 32. The second piezoelectric layer 34 includespotassium sodium niobate forming the first piezoelectric layer 32 and anelement forming the base 1. For example, when using an STO (SrTiO₃)substrate as the base 1, the second piezoelectric layer 34 may includepotassium sodium niobate, strontium, and titanium. When using Nb:STO(Nb-doped SrTiO₃) for the base 1, the second piezoelectric layer 34 mayinclude potassium sodium niobate, strontium, titanium, and niobium. Whenthe orientation control layer 6 is provided on the base 1, as shown inFIG. 4, the second piezoelectric layer 34 may include an element formingthe orientation control layer 6. For example, when using nickellanthanate for the orientation control layer 6, the second piezoelectriclayer 34 includes potassium sodium niobate, lanthanum, and nickel.

As shown in FIG. 3, the piezoelectric layer 3 may include a first-phaseportion 36 including a piezoelectric of the compositional formula(K_(a)Na_(1-a))NbO₃, and a second-phase portion 38 including apiezoelectric of the compositional formula (K_(a)Na_(1-a))_(x)NbO₃(wherein 1<x).

It is preferable that the piezoelectric layer 3 according to thisembodiment be pseudocubic (100) preferentially oriented.

The thickness of the piezoelectric layer 3 is typically selecteddepending on the application of the piezoelectric laminate 100. Thethickness of the piezoelectric layer 3 is typically 300 nanometers to3.0 micrometers. Note that the upper limit of the thickness of thepiezoelectric layer 3 may be increased up to about 10 micrometersinsofar as the density and the crystal orientation of the thin layer aremaintained.

The electrode 4 may be formed of a metal layer or a conductive complexoxide layer. The electrode 4 may be a laminate of a metal layer and aconductive complex oxide layer. As the material for the electrode 4, ametal layer formed of platinum, iridium, aluminum, or the like or aconductive complex oxide layer formed of iridium oxide or the like maybe used.

The first piezoelectric laminate 100 according to this embodiment may beformed as follows, for example.

(1) The base 1 is provided. The base 1 is selected depending on theapplication of the piezoelectric laminate 100, as described above. Asthe base 1, an STO (SrTiO₃) substrate, an Nb:STO (Nb-doped SrTiO₃)substrate, a sapphire substrate, or the like may be used.

(2) As shown in FIG. 1, the piezoelectric layer 3 including apiezoelectric of the above compositional formula is formed on the base1.

When forming the piezoelectric layer 3 using a sol-gel method or an MODmethod, the piezoelectric layer 3 may be formed by applying a coatinglayer using a precursor solution which provides a composition of theabove compositional formula, and crystallizing the coating layer.

The precursor solution as the material for forming the piezoelectriclayer 3 may be prepared by mixing organometallic compounds respectivelycontaining the constituent metal of the piezoelectric material formingthe piezoelectric layer 3 so that the metals are contained at a desiredmolar ratio, and dissolving or dispersing the organometallic compoundsusing an organic solvent such as an alcohol. As the organometalliccompounds respectively containing the constituent metal of thepiezoelectric material, an organometallic compound such as a metalalkoxide, an organic acid salt, or a beta-diketone complex may be used.Specific examples of the piezoelectric material are given below.

As the organometallic compound containing sodium (Na), sodium ethoxideand the like can be given. As the organometallic compound containingpotassium (K), potassium ethoxide and the like can be given. As theorganometallic compound containing niobium (Nb), niobium ethoxide andthe like can be given. The organometallic compounds respectivelycontaining the constituent metal of the piezoelectric material are notlimited to the above compounds. Known compounds may also be used.

Various additives such as a stabilizer may optionally be added to theprecursor solution. When causing the precursor solution to undergohydrolysis and polycondensation, an acid or a base may be added to theprecursor solution as a catalyst together with an appropriate amount ofwater.

The raw material solution is prepared so that the piezoelectric layer 3has a desired composition ratio. The piezoelectric layer 3 can be formedby applying the raw material solution to the base 1 and crystallizingthe coating layer by heat treatment. In more detail, a series of stepsincluding a raw material solution application step, a solvent (e.g.alcohol) removal step, a coating layer drying heat treatment step, and acleaning heat treatment step is performed a desired number of times, andthe resulting product is fired by crystallization annealing to form thepiezoelectric layer 3. The piezoelectric layer 3 may also be formed byperforming a series of steps including the above application step,solvent removal step, coating layer drying heat treatment step, cleaningheat treatment step, and crystallization annealing step a desired numberof times.

(3) As shown in FIG. 1, the electrode 4 is formed on the piezoelectriclayer 3. A metal layer or a conductive complex oxide layer forming theelectrode 4 is formed by known sputtering or the like.

(4) If necessary, post annealing may be performed in an oxygenatmosphere by utilizing rapid thermal annealing (RTA) or the like. Thisprovides an excellent interface between the electrode 4 and thepiezoelectric layer 3, and improves the crystallinity of thepiezoelectric layer 3.

When forming the piezoelectric laminate 100 including the orientationcontrol layer 6 on the base 1, as shown in FIG. 4, the orientationcontrol layer 6 is formed on the base 1 after the step (1). When usingnickel lanthanate for the orientation control layer 6, the orientationcontrol layer 6 may be formed by sputtering. The piezoelectric layer 3can exhibit a higher crystallinity and orientation due to the crystalstructure of the orientation control layer 6 as a result of forming theorientation control layer 6.

The first piezoelectric laminate 100 according to this embodiment can bemanufactured by the above-described steps.

The piezoelectric layer 3 includes a piezoelectric of the compositionalformula (K_(a)Na_(1-a))_(x)NbO₃ by forming the piezoelectric layer 3 asdescribed above. This piezoelectric is a perovskite oxide having anorthorhombic crystal structure at room temperature.

1.2. Second Piezoelectric Laminate

FIG. 5 is a cross-sectional view schematically showing an example of asecond piezoelectric laminate 200 according to this embodiment.

The second piezoelectric laminate 200 includes the base 1, a firstelectrode (lower electrode) 2 formed on the base 1, the piezoelectriclayer 3 formed on the lower electrode 2, and a second electrode (upperelectrode) 4 formed on the piezoelectric layer 3.

The base 1 is selected depending on the application of the piezoelectriclaminate 200. The material and the structure of the base 1 are notparticularly limited. As the base 1, the substrate described for thefirst piezoelectric laminate 100 may be used.

As the lower electrode 2, a metal layer formed of platinum or the likeor a conductive complex oxide layer may be used. A conductive layerhaving a multilayer structure in which a metal layer and a conductivecomplex oxide layer are stacked may also be used as the lower electrode2. The uppermost layer of the lower electrode 2 may be a conductivelayer which functions as a buffer layer. The buffer layer may have acrystal structure similar to that of the piezoelectric layer 3 in thesame manner as in the first piezoelectric laminate 100. When the bufferlayer has such a structure, the piezoelectric layer 3 is provided with acrystal structure similar to the crystal structure of the buffer layer.

As shown in FIG. 6, the orientation control layer 6 may optionally beprovided on the lower electrode 2. The orientation control layer 6 isthe same as that described for the first piezoelectric laminate 100. Theorientation control layer 6 is called a buffer layer or a seed layer,and has a function of controlling the crystal orientation of thepiezoelectric layer 3.

The piezoelectric layer 3 is the same as the piezoelectric layer 3 ofthe first piezoelectric laminate 100. Specifically, the piezoelectriclayer 3 includes a piezoelectric of the compositional formula(K_(a)Na_(1-a))_(x)NbO₃. In the above compositional formula, “a” ispreferably 0.1<a<1, and more preferably 0.2≦a≦0.7, and “x” is preferably1≦x≦1.2, and more preferably 1<x≦1.1. The piezoelectric of thecompositional formula (K_(a)Na_(1-a))_(x)NbO₃ has an orthorhombiccrystal structure at room temperature. If “a” in the above compositionalformula is within the above range, the phase change temperature from anorthorhombic crystal to a rhombohedral crystal (a≦0.55) and the phasechange temperature from an orthorhombic crystal to a monoclinic crystal(0.55≦a) are decreased to −40° C. or less, whereby stablecharacteristics can be obtained in a low temperature region. Itemsregarding the ranges of “a” and “x” in the compositional formula and thefeatures of the piezoelectric layer 3 are the same as described for thefirst piezoelectric laminate 100. Therefore, detailed descriptionthereof is omitted.

When the piezoelectric layer has a structure corresponding to FIG. 2,the piezoelectric layer 3 may include the first piezoelectric layer 32including a piezoelectric of the compositional formula(K_(a)Na_(1-a))NbO₃, and the second piezoelectric layer 34 formedbetween the first piezoelectric layer 32 and the base 1 and including apiezoelectric (potassium sodium niobate containing potassium and/orsodium in excess) containing at least an element forming a layer (lowerelectrode 2 in the example shown in FIG. 5) contacting the piezoelectriclayer 3.

When the piezoelectric layer has a structure corresponding to FIG. 3,the piezoelectric layer 3 may include the first-phase portion 36including a piezoelectric of the compositional formula(K_(a)Na_(1-a))NbO₃, and the second-phase portion 38 including apiezoelectric of the compositional formula (K_(a)Na_(1-a))_(x)NbO₃(wherein 1<x).

The upper electrode 4 may be formed of a metal layer, a conductivecomplex oxide layer, or a laminate of a metal layer and a conductivecomplex oxide layer in the same manner as the lower electrode 2.Specifically, a metal layer formed of platinum, iridium, or the like ora conductive complex oxide layer formed of iridium oxide or the like maybe used as the upper electrode 4.

The second piezoelectric laminate 200 according to this embodiment maybe formed as follows, for example.

(1) A base 1 is provided. As a base 1, the base 1 described for thefirst piezoelectric laminate 100 may be used. For example, a siliconsubstrate may be used as the base 1.

(2) As shown in FIG. 5, the lower electrode 2 is formed on the base 1. Ametal layer or a conductive complex oxide layer forming the lowerelectrode 2 is formed by known sputtering or the like.

(3) As shown in FIG. 5, the piezoelectric layer 3 of the abovecompositional formula is formed on the lower electrode 2. The method offorming the piezoelectric layer 3 is the same as that for the firstpiezoelectric laminate 100. Therefore, detailed description thereof isomitted.

(4) As shown in FIG. 5, the upper electrode 4 is formed on thepiezoelectric layer 3. The structure and the formation method of a metallayer or a conductive complex oxide layer forming the upper electrode 4are the same as those of the electrode 4 of the first piezoelectriclaminate 100. Therefore, detailed description thereof is omitted.

(5) If necessary, post annealing may be performed in an oxygenatmosphere utilizing RTA or the like. This provides an excellentinterface between each of the lower electrode 2 and the upper electrode4 and the piezoelectric layer 3, and improves the crystallinity of thepiezoelectric layer 3.

When forming the piezoelectric laminate 200 including the orientationcontrol layer 6 on the lower electrode 2, as shown in FIG. 6, theorientation control layer 6 is formed on the lower electrode 2 after thestep (2). When using nickel lanthanate for the orientation control layer6, the orientation control layer 6 may be formed by sputtering. Thepiezoelectric layer 3 can exhibit a higher crystallinity and orientationdue to the crystal structure of the orientation control layer 6 as aresult of forming the orientation control layer 6.

The second piezoelectric laminate 200 according to this embodiment canbe manufactured by the above-described steps.

The piezoelectric layer 3 may be formed using a liquid phase method suchas a sol-gel method or a metal organic decomposition (MOD) method or avapor phase method such as a laser ablation method or a sputteringmethod.

The first and second piezoelectric laminates 100 and 200 include thepiezoelectric layer 3 with excellent piezoelectric characteristics, andmay be suitably applied to various applications described later.

2. EXAMPLES

Examples according to the invention are described below. Note that theinvention is not limited to the following examples.

2.1. Example 1

Potassium ethoxide, sodium ethoxide, and niobium ethoxide were mixed ata molar ratio of K:Na:Nb=1.0:0.2:1.0. The mixed liquid was refluxed inbutyl cellosolve to prepare a triple alkoxide solution. Then,diethanolamine was added to the solution as a stabilizer. A precursorsolution was thus prepared. Note that acetic acid may be used instead ofdiethanolamine. The precursor solution was applied to a base on which aplatinum layer was formed (platinum layer/silicon oxide layer/siliconsubstrate) by spin coating, dried and prefired on a hot plate, andsubjected to rapid thermal annealing at 700° C. This step was repeatedlyperformed several times to obtain a polycrystalline potassium sodiumniobate (KNN) layer with a thickness of about 1.5 micrometers. Aplatinum electrode with a thickness of 100 nanometers and a diameter of200 micrometers was formed on the KNN layer by sputtering. A capacitorsample was thus obtained.

Hysteresis characteristics were evaluated using the above capacitorsample. A hysteresis loop shown in FIG. 7 was obtained. As shown in FIG.7, it was confirmed that the capacitor sample of this example exhibitsexcellent hysteresis characteristics and the KNN layer exhibitsferroelectricity.

2.2. Example 2 and Comparative Example 1

KNN layer forming precursor solutions were prepared in the same manneras in Example 1 except for changing the molar ratio (mol %) of sodium topotassium as shown in Table 1. Specifically, the ratio of sodium (amountof excess Na) to potassium in the precursor solution was adjusted to 10mol %, 20 mol %, 40 mol %, and 50 mol %. The resulting precursorsolutions were applied to an Nb:STO (Nb-doped SrTiO₃) single crystalsubstrate by spin coating, dried and prefired on a hot plate, andsubjected to rapid thermal annealing at 700° C. This step was repeatedlyperformed several times to obtain four types of polycrystalline KNNlayers with a thickness of about 1 micrometer.

TABLE 1 Amount of excess Na Composition in solution (mol %)(K_(a)Na_((1−a)))_(x)NbO₃ 10 x = 1.04, a = 0.85 20 x = 1.09, a = 0.79 40x = 1.08, a = 0.63 50 x = 1.08, a = 0.56

As Comparative Example 1, a potassium niobate layer (KN layer) wasformed in the same manner as in Example 2 except that sodium was notadded to the precursor solution and potassium ethoxide and niobiumethoxide were mixed at a molar ratio of K:Nb=1.0:1.0.

The resulting KNN layer and KN layer were evaluated as follows.

(1) Compositional Analysis of KNN Layer

The composition of the KNN layer according to Example 2 was analyzed byinduction-coupled plasma (ICP) emission spectrometry. The results areshown in Table 1. As shown in Table 1, it was confirmed that the KNNlayer according to the example had a value for x in the formula(K_(a)Na_(1-a))_(x)NbO₃ greater than one, that is, K and Na werecontained in excess of Nb in comparison with the stoichiometriccomposition. The value “x” was about 1.1 at maximum even when the amountof Na in the precursor solution was increased.

(2) Surface Observation by SEM

The surfaces of two KNN layers (x=1.04 and 1.09) according to Example 2and the KN layer according to Comparative Example 1 were observed byscanning electron microscopy (SEM). The results are shown in FIGS. 8A to8C. FIGS. 8A and 8B show the results of Example 2, and FIG. 8C shows theresults of Comparative Example 1. As shown in FIGS. 8A and 8B, it wasconfirmed that the KNN layer according to Example 2 exhibited ahomogeneous and excellent morphology. As shown in FIG. 8C, it wasconfirmed that a heterophase was formed in the KN layer according toComparative Example 1.

(3) Crystallinity Determined by XRD

The crystallinity of the samples used in (2) was examined by X-raydiffraction (XRD) analysis. The results are shown in FIG. 9. In FIG. 9,the charts indicated by the symbols “a” and “b” show the results of theKNN layers according to Example 2, and the chart indicated by the symbol“c” shows the results of the KN layer according to Comparative Example1.

As shown in FIG. 9, it was confirmed that the KNN layer according toExample 2 exhibited excellent crystallinity and was (100)-oriented. Onthe other hand, it was confirmed that the KN layer according toComparative Example 1 showed a peak of a heterophase (K₄Nb₆O₁₇) andexhibited poor crystallinity.

(4) Raman Spectroscopy

The KNN layers obtained in Example 2 (i.e. KNN layers obtained using theprecursor solutions in which the ratio of sodium (amount of excess Na)to potassium was adjusted to 10 mol %, 20 mol %, 40 mol %, and 50 mol %)and a KNN layer obtained in the same manner as in Example 2 using aprecursor solution in which the amount of excess Na was adjusted to 30mol % were subjected to Raman spectroscopy. The results are shown inFIG. 12. In FIG. 12, the symbols “a” to “e” respectively show thespectra when the amount of excess Na was 10 mol % to 50 mol %.

As is clear from the spectra shown in FIG. 12, the first peal (peakpresent between 500 and 550 cm⁻¹) attributed to the A site of KNN isshifted and the second peak near 600 cm⁻¹ is broadened depending on theamount of excess Na. Therefore, it was confirmed that potassium andsodium were present in the A site.

(5) Spacing Determined from X-Ray Analysis

The spacing in the (100) plane of each sample according to Example 2 wasdetermined from the X-ray analysis (theta-2theta) peak. The results areshown in FIG. 13.

As shown in FIG. 13, it was confirmed that the spacing (d(100))decreases as the amount of excess Na increases. Specifically, since theatomic radius of sodium is smaller than that of potassium, the spacingdecreases as the amount of sodium added increases. This also confirmsthat potassium and sodium were present in the A site.

2.3. Example 3

A KNN layer forming precursor solution was prepared in the same manneras in Example 1. The resulting precursor solution was applied to anNb:STO (Nb-doped SrTiO₃) single crystal substrate by spin coating, driedand prefired on a hot plate, and subjected to rapid thermal annealing at700° C. This step was repeatedly performed several times to obtain apolycrystalline KNN layer with a thickness of about 1 micrometer. Aplatinum electrode with a thickness of 100 nanometers and a diameter ofabout 30 micrometers was formed on the KNN layer using a lift-offmethod. The platinum layer was formed by sputtering. The substrate wasbonded to a platinum-coated silicon substrate through a silver paste.

The sample thus obtained was subjected to field-strain measurement usingan atomic force microscope (AFM). The results are shown in FIG. 10. Thefield-strain curve shown in FIG. 10 indicates that the KNN layeraccording to this example produces a piezoelectric vibration due toapplication of voltage. It was also confirmed that the hysteresis curveobserved in Example 1 originates in the piezoelectric characteristics ofthe KNN layer.

2.4. Example 4

A KNN layer forming precursor solution was prepared in the same manneras in Example 1. The resulting sol solution was applied to an STO(SrTiO₃) single crystal substrate by spin coating, dried and prefired ona hot plate, and subjected to rapid thermal annealing at 700° C. Thisstep was repeatedly performed several tens of times to obtain apolycrystalline KNN layer with a thickness of about 10 micrometers.After planarizing the KNN layer by CMP, an aluminum layer with athickness of 100 nanometers was deposited on the INN layer, and atwo-port comb electrode (L/S=5 micrometers) was formed byphotolithography. An S21 parameter indicating SAW propagationcharacteristics was measured using a network analyzer.

FIG. 11 shows the S21 parameter measurement results. The resonancewaveform (indicated by the symbol “a”) of the KNN/STO substrate with aspecific bandwidth of about 15% was observed near 240 MHz. As shown inFIG. 11, it was confirmed that the KNN/STO substrate according to thisexample excites surface acoustic waves due to stress.

2.5. Example 5 and Reference Example 1

Potassium ethoxide, sodium ethoxide, and niobium ethoxide were mixed ata molar ratio of K:Na:Nb=1.0:0.2:1.0. The mixed liquid was refluxed inbutyl cellosolve to prepare a triple alkoxide solution. Then,diethanolamine was added to the solution as a stabilizer. A precursorsolution was thus prepared. Note that acetic acid may be used instead ofdiethanolamine. The precursor solution was applied to a substrate onwhich a polycrystalline nickel lanthanate (LNO) layer was formed as anorientation control layer (nickel lanthanate layer/platinumlayer/silicon oxide layer/silicon substrate) by spin coating, dried andprefired on a hot plate, and subjected to rapid thermal annealing at700° C. This step was repeatedly performed eight times to obtain apolycrystalline potassium sodium niobate (KNN) layer with a thickness ofabout 1 micrometer.

The surface flatness of the sample was evaluated by SEM. The results areshown in FIG. 14A. As is clear from these results, it was confirmed thatthe KNN layer according to this example was dense and exhibitedexcellent crystallinity due to the crystallinity of the LNO layer(orientation control layer). The crystallinity of the sample wasdetermined by XRD. The results are shown in FIG. 15A. As is clear fromthese results, it was confirmed that the KNN layer according to thisexample was (100)-single-oriented even at a thickness of 1 micrometer.

An S parameter indicating SAW propagation characteristics was observedfor the KNN layer of Example 5. The propagation loss was as small as −1db or less, which is applicable to SAW devices and the like.

As Reference Example 1, a KNN layer was formed in the same manner as inExample 5 except that the orientation control layer was not formed. Thesurface flatness of this KNN layer was evaluated by SEM. The results areshown in FIG. 14B. As is clear from these results, since the KNN layeraccording to this example did not include an LNO layer as theorientation control layer, the surface flatness of the KNN layer waspoor in comparison with Example 5. The crystallinity of the KNN layerwas examined by XRD. The results are shown in FIG. 15B. As is clear fromthese results, it was confirmed that the KNN layer is randomly orientedwhen forming the KNN layer on the platinum layer.

3. APPLICATION EXAMPLE 3.1. Surface Acoustic Wave Device

An example of a surface acoustic wave device to which the firstpiezoelectric laminate 100 according to the invention is applied isdescribed below with reference to the drawings.

FIG. 16 is a cross-sectional view schematically showing a surfaceacoustic wave device 300 according to this embodiment.

The surface acoustic wave device 300 is formed by applying the firstpiezoelectric laminate 100 shown in FIGS. 1 and 4. Specifically, thesurface acoustic wave device 300 includes the base 1 and thepiezoelectric layer 3 formed on the base 1 of the first piezoelectriclaminate 100, and electrodes (i.e. interdigital transducers (hereinaftercalled “IDT electrodes”)) 18 and 19 formed on the piezoelectric layer 3.The IDT electrodes 18 and 19 are formed by patterning the electrode 4shown in FIG. 2.

3.2. Frequency Filter

An example of a frequency filter to which the surface acoustic wavedevice according to the invention is applied is described below withreference to the drawings. FIG. 17 is a view schematically showing thefrequency filter.

As shown in FIG. 17, the frequency filter includes a laminate 140. Asthe laminate 140, a laminate (see FIG. 16) similar to that of theabove-described surface acoustic wave device 300 may be used.Specifically, the laminate 140 may include the base 1 and thepiezoelectric layer 3 shown in FIGS. 1 and 4.

The laminate 140 includes IDT electrodes 141 and 142 formed bypatterning the electrode 4 shown in FIGS. 1 and 4 on its top surface.Sound absorbing sections 143 and 144 are formed on the top surface ofthe laminate 140 so that the IDT electrodes 141 and 142 are positionedin between. The sound absorbing sections 143 and 144 absorb surfaceacoustic waves propagated on the surface of the laminate 140. Ahigh-frequency signal source 145 is connected with the IDT electrode141, and signal lines are connected with the IDT electrode 142.

3.3. Oscillator

An example of an oscillator to which the surface acoustic wave deviceaccording to the invention is applied is described below with referenceto the drawings. FIG. 18 is a view schematically showing the oscillator.

As shown in FIG. 18, the oscillator includes a laminate 150. As thelaminate 150, a laminate (see FIG. 16) similar to that of theabove-described surface acoustic wave device 300 may be used.Specifically, the laminate 150 may include the base 1 and thepiezoelectric layer 3 shown in FIGS. 1 and 4.

An IDT electrode 151 is formed on the top surface of the laminate 150,and IDT electrodes 152 and 153 are formed so that the IDT electrode 151is positioned in between. A high-frequency signal source 154 isconnected with a comb-shaped electrode 151 a forming the IDT electrode151, and a signal line is connected with the other comb-shaped electrode151 b. The IDT electrode 151 corresponds to an electrode for applying anelectric signal, and the IDT electrodes 152 and 153 correspond toresonation electrodes for causing a specific frequency component or afrequency component in a specific band of surface acoustic wavesgenerated by the IDT electrode 151 to resonate.

The above-described oscillator may also be applied to a voltagecontrolled saw oscillator (VCSO).

As described above, the frequency filter and the oscillator may includethe surface acoustic wave device according to the invention.

3.4. Thin-Film Piezoelectric Resonator

An example of a thin-film piezoelectric resonator to which thepiezoelectric laminate according to the invention is applied isdescribed below with reference to the drawings.

3.4.1. First Thin-Film Piezoelectric Resonator

FIG. 19 is a view schematically showing a first thin-film piezoelectricresonator 700 as an example of this embodiment. The first thin-filmpiezoelectric resonator 700 is a diaphragm type thin-film piezoelectricresonator.

The first thin-film piezoelectric resonator 700 includes a substrate701, an elastic layer 703, a lower electrode 704, a piezoelectric layer705, and an upper electrode 706. The substrate 701, the lower electrode704, the piezoelectric layer 705, and the upper electrode 706 of thethin-film piezoelectric resonator 700 respectively correspond to thebase 1, the lower electrode 2, the piezoelectric layer 3, and the upperelectrode 4 of the piezoelectric laminate 200 shown in FIGS. 5 and 6. Asthe elastic layer 703, a layer such as a buffer layer not shown in FIG.5 (orientation control layer in FIG. 6) may be used. Specifically, thefirst thin-film piezoelectric resonator 700 includes the piezoelectriclaminate 200 shown in FIGS. 5 and 6.

A via-hole 702 is formed through the substrate 701. An interconnect 708is provided on the upper electrode 706. The interconnect 708 iselectrically connected with an electrode 709 formed on the elastic layer703 through a pad 710.

3.4.2. Second Thin-Film Piezoelectric Resonator

FIG. 20 is a view schematically showing a second thin-film piezoelectricresonator 800 as another example of this embodiment. The secondthin-film piezoelectric resonator 800 differs from the first thin-filmpiezoelectric resonator 700 shown in FIG. 19 in that an air gap 802 isformed between a substrate 801 and an elastic layer 803 without forminga via-hole.

The second thin-film piezoelectric resonator 800 includes a substrate801, an elastic layer 803, a lower electrode 804, a piezoelectric layer805, and an upper electrode 806. The substrate 801, the lower electrode804, the piezoelectric layer 805, and the upper electrode 806 of thethin-film piezoelectric resonator 800 respectively correspond to thebase 1, the lower electrode 2, the piezoelectric layer 3, and the upperelectrode 4 of the piezoelectric laminate 200 shown in FIGS. 5 and 6. Asthe elastic layer 803, a layer such as a buffer layer not shown in FIG.5 (orientation control layer 6 in FIG. 6) may be used. Specifically, thesecond thin-film piezoelectric resonator 800 includes the piezoelectriclaminate 200 shown in FIGS. 5 and 6. The air gap 802 is the space formedbetween the substrate 801 and the elastic layer 803.

The thin-film piezoelectric resonator according to this embodiment (e.g.first thin-film piezoelectric resonator 700 and second thin-filmpiezoelectric resonator 800) can function as a resonator, a frequencyfilter, or an oscillator.

3.5. Piezoelectric Actuator

An inkjet recording head is described below as an example in which thesecond piezoelectric laminate 200 according to the invention is appliedto a piezoelectric actuator. FIG. 21 is a cross-sectional view showing aschematic configuration of an inkjet recording head to which thepiezoelectric actuator according to this embodiment is applied, and FIG.22 is an exploded perspective view of the inkjet recording head which isillustrated in a vertically reversed state.

As shown in FIGS. 21 and 22, an inkjet recording head 50 includes a headbody 57 and a piezoelectric section 54 formed above the head body 57.The piezoelectric laminate 200 shown in FIGS. 5 and 6 is applied as thepiezoelectric section 54, in which the lower electrode 2, thepiezoelectric layer 3, and the upper electrode 4 are stacked in thatorder. In the inkjet recording head, the piezoelectric section 54functions as a piezoelectric actuator.

The head body 57 includes a nozzle plate 51, an ink chamber substrate52, and an elastic layer 55. The base 1 of the piezoelectric laminate200 shown in FIGS. 5 and 6 forms the elastic layer 55 shown in FIG. 21.As the elastic layer 55, a layer such as a buffer layer not shown inFIGS. 5 and 6 may be used. The base 1 of the piezoelectric laminate 200also forms the ink chamber substrate 52 shown in FIG. 21. A cavity 521is formed in the ink chamber substrate 52. A nozzle 511 continuous withthe cavity 521 is formed in the nozzle plate 51. As shown in FIG. 22,these members are accommodated in a housing 56 to form the inkjetrecording head 50. The diameter of the nozzle 511 is 10 to 30micrometers. The nozzles 511 are formed at a pitch of 90 to 300 nozzlesper inch.

Each piezoelectric section is electrically connected with apiezoelectric device driver circuit (not shown), and is actuated(vibrate or displaced) based on a signal from the piezoelectric devicedriver circuit. Specifically, each piezoelectric section 54 functions asa vibration source (head actuator). The elastic layer 55 vibrates due tovibration (deflection) of the piezoelectric section 54, and functions tomomentarily increase the pressure inside the cavity 521. The maximumvoltage applied to the piezoelectric is 20 to 40 V. The piezoelectric isdriven at 20 to 50 kHz. The amount of ink discharged is typically 2 to 5picoliters.

The inkjet recording head which discharges ink has been described aboveas an example. Note that this embodiment aims at a liquid jetting headutilizing a piezoelectric laminate as a piezoelectric actuator. As theliquid jetting head, a recording head used for an image recording devicesuch as a printer, a color material jetting head used to produce a colorfilter for a liquid crystal display or the like, an electrode materialjetting head used to form an electrode for an organic EL display, afield emission display (FED), or the like, a bio-organic substancejetting head used to produce a bio-chip, and the like can be given.

The invention is not limited to the above-described embodiments, andvarious modifications can be made. For example, the invention includesvarious other configurations substantially the same as theconfigurations described in the embodiments (in function, method andresult, or in objective and result, for example).

The invention also includes a configuration in which an unsubstantialportion in the described embodiments is replaced. The invention alsoincludes a configuration having the same effects as the configurationsdescribed in the embodiments, or a configuration able to achieve thesame objective. Further, the invention includes a configuration in whicha publicly known technique is added to the configurations in theembodiments.

Although only some embodiments of the invention have been described indetail above, those skilled in the art will readily appreciate that manymodifications are possible in the embodiments without materiallydeparting from the novel teachings and advantages of this invention.Accordingly, all such modifications are intended to be included withinthe scope of the invention.

1. A piezoelectric film comprising (K_(a)Na_(1-a))_(x)NbO₃, wherein “a” is 0.1<a<1 and “x” is 1≦x≦1.2.
 2. The piezoelectric film as defined in claim 1, wherein “x” is 1<x≦1.1.
 3. The piezoelectric film according to claim 1, wherein “a” is 0.2≦a≦0.7.
 4. The piezoelectric film according to claim 2, wherein “a” is 0.2≦a≦0.7.
 5. The piezoelectric film according to claim 1, wherein (K_(a)Na_(1-a))_(x)NbO₃ includes pseudocubic (100) orientation.
 6. The piezoelectric film according to claim 2, wherein (K_(a)Na_(1-a))_(x)NbO₃ includes pseudocubic (100) orientation.
 7. The piezoelectric film according to claim 3, wherein (K_(a)Na_(1-a))_(x)NbO₃ includes pseudocubic (100) orientation.
 8. The piezoelectric film according to claim 4, wherein (K_(a)Na_(1-a))_(x)NbO₃ includes pseudocubic (100) orientation.
 9. A surface acoustic wave device comprising the piezoelectric film as defined in claim
 1. 10. A piezoelectric resonator comprising the piezoelectric film as defined in claim
 1. 